Plastic lead frames for semiconductor devices, packages including same, and methods of fabrication

ABSTRACT

A conductive plastic lead frame and method of manufacturing same suitable for use in IC packaging. In a preferred embodiment, the lead frame is constructed of a plastic or polymer based lead frame structure with an intrinsic conductive polymer coating. In a second embodiment the lead frame is a composite plastic or polymeric material intermixed with an intrinsic conductive polymer coating.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation of application Ser. No.09/639,359, filed Aug. 14, 2000, pending which is a continuation ofapplication Ser. No. 09/195,765, filed Nov. 18, 1998, now U.S. Pat. No.6,124,151, issued Sep. 26, 2000, which is a continuation of applicationSer. No. 08/878,935, filed Jun. 19, 1997, now U.S. Pat. No. 5,879,965,issued Mar. 9, 1999.

BACKGROUND OF THE INVENTION

[0002] Field of the Invention: The present invention relates tointegrated circuit lead frames and methods of production thereof. Inparticular, this invention relates to plastic lead frames with aconductive coating or material contained therein used for packagingintegrated circuits and methods of manufacturing the same.

[0003] State of the Art: Integrated circuit (IC) chips are enclosed inplastic packages that provide protection from hostile environments andenable electrical interconnection to printed circuit boards. During amanufacturing process, the IC chip is typically attached to a die paddleof a conventional lead frame or suspended from the lead fingers of aleads-over-chip (LOC) lead frame using an adhesive such as epoxy ordouble-sided tape, and subsequently encapsulated with a dense and rigidplastic by a transfer molding process. In essence, the lead frame formsthe backbone of the molded plastic IC package.

[0004] Lead frames typically perform many functions such as: (1) aholding fixture that indexes with tool-transfer mechanisms as thepackage proceeds through various assembly operations, (2) a dam thatprevents plastic from rushing out between leads during the moldingoperation, (3) a chip attach substrate, (4) a support matrix for theplastic, and (5) an electrical and thermal conductor from chip to board.

[0005] Traditionally, lead frames are fabricated from a strip of sheetmetal by stamping or chemical milling operations. There are manydifferent metal alloy compositions which are commercially available forproducing lead frames. For example, Rao R. Tummala and Eugene J.Rymaszewski, “Microelectronics Packaging Handbook,” Table 8-4, 1989,provides 16 different alloys available from 9 different manufacturers.Lead frame material selection depends on many factors such as cost, easeof fabrication, strength, thermal conductivity, and matched coefficientof thermal expansion (CTE). A close match of CTE between the silicon dieand the lead frame is required to avoid chip fracture from differentialexpansion rates.

[0006] The most widely used metal for lead frame fabrication is Alloy 42(42% Nickel-58% Iron). Alloy 42 has a CTE near silicon and good tensilestrength properties. The disadvantage of Alloy 42 is that it has lowthermal conductivity. Since the lead frame is the main conduit by whichheat flows from the chip to the environment and printed circuit board,this can have a profound effect on the package thermal resistance afterprolonged device operation.

[0007] A layered composite strip, such as copper-clad stainless steel,was developed to emulate the mechanical properties of Alloy 42 whileincreasing thermal conductivity. However, copper-clad stainless steel issomewhat more expensive to manufacture than Alloy 42. When manufacturingcopper-clad stainless steel lead frames, the cladding is accomplished byhigh-pressure rolling of copper foil onto a stainless steel strip,followed by annealing the composite to form a solid-solution weld. Whilecopper alloys provide good thermal conductivity and have a CTE near thatof low-stress molding compounds, there is a substantial CTE mismatchwith respect to silicon.

[0008] While numerous alloys have been developed to solve problems withthermal conductivity, CTE mismatch, and strength, other importantfactors such as ease of fabrication and cost have not improved asreadily.

[0009] Conventional methods for making lead frames for integratedcircuit devices are described in U.S. Pat. No. 3,440,027. The use of aplastic support structure in a method of forming metal lead frames isdescribed in U.S. Pat. No 4,089,733 (hereinafter the ′″733″ patent). Theplastic support structure of the '733 patent solves the problem ofdeformed and misaligned lead fingers resulting from stress during thebonding process by supporting the lead fingers with a plastic structure.However, the '733 patent requires a metal lead frame in addition to theplastic support structure with its attendant costs. A method ofmanufacturing multi-layer metal lead frames is disclosed in U.S. Pat.No. 5,231,756 (hereinafter the ′″756″ patent). The '756 patent providesan improvement in aligning power and ground planes for use in amulti-layer lead frame where such planes are necessary. However, thenumber of steps required to manufacture such multi-layer lead frameswill not solve the problem of decreasing costs. In short, none of therelated art appears to disclose methods of producing low-cost leadframes made from materials not structurally based on metal.

[0010] Since packaged ICs are produced in high volumes, a small decreasein the cost per packaged IC can result in substantial savings overall.Accordingly, there is a need in the industry for a low cost plastic leadframe with suitable characteristics for IC packaging.

BRIEF SUMMARY OF THE INVENTION

[0011] The present invention comprises plastic lead frames coated withconductive materials or having conductive materials therein suitable foruse in IC packaging, and methods for fabricating same. The invention maybe used in the production of ICs.

[0012] By using plastic as the structural base for a lead frame, manycosts associated with the manufacture of metal lead frames can beeliminated. For instance, plastic lead frames can be injection molded orstamped and then coated with an intrinsic conductive polymer.Furthermore, plastic is intrinsically less expensive as a bulk materialthan metal alloys typically used in lead frame construction.

[0013] Once the lead frame is formed, it can be used in either theconventional die attach and connect process or in a LOC process. In theconventional process, the die is adhesively attached to a die paddleusing epoxy or double-sided tape, followed by wire bonding where diepads are connected to lead fingers from the lead frame. In a LOCprocess, the die could be attached to the lead frame fingers bydouble-sided adhesive tape followed by wire bonding. Alternatively, theLOC process might include direct connection between the die pads and theLOC lead frame fingers with a conductive epoxy or Z-axis conductivematerial using methods common in the art.

[0014] In the preferred embodiment of the invention, a plastic leadframe coated with an intrinsically conductive polymer is provided. Theplastic lead frame structure can be formed by injection molding,stamping or etching from a sheet of plastic or polymer material. Thisplastic lead frame structure is then coated with a conductive polymer bydipping in a solution or lacquer composed of a polyaniline such as thecommercially available product Ormecon™. By controlling the polyanilinecoating process, precise layers with known thicknesses can be produced.The resulting low cost lead frame has a conductive layer surrounding theplastic structure. Moreover, the CTE of silicon, conductive polymer, andmolding compounds is very nearly matched.

[0015] Another embodiment of the present invention is a compositeplastic lead frame formed of a conventional polymer intermixed with aconductive polymer. The composite plastic lead frame structure is formedby injection molding, stamping or etching from a sheet of the compositeplastic/conductive polymer material. The CTE of the lead frame is wellmatched to that of silicon as in the preferred embodiment; however,increased quantities of polyaniline required to provide sufficientconductivity may consequently increase cost relative to the preferredembodiment.

[0016] Additional advantages of both the above embodiments aretransparency, corrosion resistance, and oxidation resistance.Polyaniline is transparent. By using transparent plastic or polymer inthe lead frame structure, ultraviolet (UV), or other light source, cureof the die attach material becomes possible. This is particularlyadvantageous in an automated production environment. Furthermore, bothof the above lead frame embodiments are nonmetallic and thus lesssusceptible to corrosion or oxidation.

[0017] The inventive plastic lead frames solve the problem of reducingcost while maintaining characteristics necessary for use in commercialproduction of IC packages. The overall cost of IC chip packaging isreduced by using plastic lead frames coated with conductive layers. Useof transparent polymers and intrinsically conductive polymersfacilitates UV or other light source cure of die attach materials.Furthermore, the methods used to produce such lead frames are simple andcan be easily incorporated into existing high-speed production lines formanufacturing IC chips. While the inventive plastic lead framesdescribed herein have been illustrated with respect to conventional wirebonding and LOC interconnect technology, there is theoretically nolimitation to applying the invention to conventional lead frames,emerging tape automated bonding (TAB) technology, etc. as well.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0018]FIG. 1 is a perspective view of the inventive lead frame in a LOCconfiguration with direct attachment to IC die pads;

[0019]FIG. 2 is a blow-up cross-sectional view of the inventive leadframe as it would be attached to an input/output pad of an IC die in theLOC configuration;

[0020]FIG. 3 is a perspective view of the inventive lead frame in a LOCconfiguration where the IC die is adhesively attached to the leadfingers and wire bonded to die bond pads;

[0021]FIG. 4 is a cross-sectional view of the preferred embodimentshowing the plastic frame structure with a coating of polyaniline ofthickness “d”;

[0022]FIG. 5 is a perspective view of the inventive lead frame in aconventional lead frame configuration with wire bond attachment from ICbond pads to the lead fingers of the lead frame;

[0023]FIG. 6 is a cross-sectional view of an embodiment of the presentinvention of a LOC type lead frame in an encapsulated package mounted ona substrate; and

[0024]FIG. 7 is a cross-sectional view of another embodiment of thepresent invention of a conventional type lead frame in an encapsulatedpackage mounted on a substrate.

DETAILED DESCRIPTION OF THE INVENTION

[0025] An understanding of the detailed description of the invention isfacilitated by reference to the drawings, FIGS. 1 through 5. Each of thefour embodiments of the invention solve the problem of reducing cost ofproducing lead frames for IC chip packaging. Additionally, at least twoof the embodiments improve the following characteristics: CTE matchingof the lead frame, silicon, and adhesive, anti-corrosion,anti-oxidation, and in-line cure of the die attach adhesive.

[0026] Drawing FIG. 1 shows the preferred embodiment of the inventiveplastic lead frame 10 as envisioned for application in a LOC packagingconfiguration. The lead fingers 12 of the plastic lead frame 10 (notcompletely shown) are positioned over die bond pads 14. The lead fingers12 are directly connected to the die bond pads 14 by an adhesive 16consisting of a conductive epoxy or Z-axis conductive material. The ICchip or die 18 is suspended by the adhesive 16 connecting the leadfingers 12 of the plastic lead frame 10.

[0027] Drawing FIG. 2 depicts an enlarged cross-sectional view of thepreferred embodiment showing a lead finger 12 as attached to a die bondpad 14 on the IC die 18. The cross-section of the lead finger 12 is alsoshown with the inner lead frame structure 20 coated with a conductivecoating 22. The conductive coating 22 could be a conductive epoxy,Z-axis conductive material, or any other suitably conductive adhesiveknown in the art. The die bond pad 14 is connected to a circuit trace 24leading to components (not shown) on the IC die 18. The circuit trace 24will typically be underneath a passivation layer of oxide 26 on the ICdie 18.

[0028] Drawing FIG. 3 shows a perspective view of the preferredembodiment in a LOC configuration which utilizes conventional wirebonding. In drawing FIG. 3, lead fingers 12 of the plastic lead frame 10(not completely shown) are located over an adhesive tape 28 which holdsthe IC die 18 to the plastic lead frame 10. Die bond pads 14 areconnected to lead fingers 12 by means of wire 30. The wire 30 can bealuminum or gold and is attached using wire bonding machines (not shown)well established in the art.

[0029] Drawing FIG. 4 shows a magnified cross-section of a plastic leadfinger 12 of the preferred embodiment of the inventive plastic leadframe 10 (not shown). The inner plastic lead frame structure 20 is madeof a conventional plastic or polymer material. The surroundingconductive coating 22 is an intrinsic conductive polymer, such aspolyaniline, or copper. The polyaniline layer is of thickness “d”. Theminimum thickness “d” necessary for suitable electrical conductivity isgoverned by the following equation:

d=1/(πƒσμ)^(½)

[0030] where f is the maximum frequency of the electrical device, , isthe permeability of the polyaniline layer, and σ is the conductivity ofthe polyaniline layer. For example, where f is 1×10⁹Hz, σ is 1×10⁵(Ohm·m)⁻¹ and μ is 1.26×10⁻⁶ Henry/m, a thickness of 50 μm is needed forthe polyaniline coating.

[0031] Drawing FIG. 5 shows a perspective view of the preferredembodiment in a conventional type lead frame configuration whichutilizes conventional wire bonding. In drawing FIG. 5, lead fingers 112of the plastic lead frame 100 (not completely shown) are locatedadjacent the sides 116 which hold the IC die 118 to the lead frame 100.Die bond pads 114 are connected to lead fingers 112 by means of wires130. The wires 130 can be aluminum or gold and is attached using wirebonding machines (not shown) well established in the art. The IC die 118is supported by the die paddle 120 of the lead frame 110 and isadhesively secured thereto by means of a suitable epoxy adhesive or,alternately, by means of a double-sided adhesively coated tape.

[0032] Drawing FIG. 6 shows an IC die 218 encapsulated by material 230connected to a LOC type lead frame having lead fingers 212 connected tothe bond pads on the active surface of the IC die 218 and connected toelectrical circuits (not shown) on a substrate 220, such as a printedcircuit board or the like. The lead fingers 212 may be shaped in anysuitable type configuration for connection to the IC die 218 and theelectrical circuits of substrate 220. The encapsulating material 230 maybe of any well known suitable type and may include suitable fillermaterial therein.

[0033] Drawing FIG. 7 shows an IC die 318 encapsulated by material 340connected to a conventional type lead frame having lead fingers 312 anda die paddle 316 supporting the IC die 318. The lead fingers 312 areconnected to the bond pads 314 on the active surface of the IC die 318by wires 330 and are connected to electrical circuits (not shown) on asubstrate 320. The lead fingers 312 may be shaped in any suitable typeconfiguration for connection to the IC die 318 and the electricalcircuits of substrate 320. The encapsulating material 340 may be anywell known suitable type and may include suitable filler materialtherein.

[0034] In the preferred embodiment, plastic lead frames can be dipped inan intrinsically conductive polymer, such as polyaniline, to form aconductive layer. The polyaniline dip could be a dispersion ofpolyaniline in a mixture of organic solvents. The coating is finished bydrying the coating with infrared heating or baking using techniques wellknown in the art. This dip coating process can be repeated as necessaryto attain the desired thickness. Alternatively, the polyaniline can beapplied in a lacquer dispersion, again using techniques well known inthe art.

[0035] In a second embodiment, the polyaniline could be dispersedthroughout the plastic lead frame structure using commercially availablepolyaniline polymer dispersions such as those offered by ZipperlingKessler & Co. The composite lead frame could be formed by injectionmolding or stamping a sheet of the composite polymer containingpolyaniline.

[0036] In both the preferred and second embodiment, such plastic leadframes described have less CTE mismatch with respect to the silicon ICdie, plastic mold compounds, and die attach materials relative totraditional metal alloys. Furthermore, conductive plastic lead framesare more flexible than metal alloy lead frames to resist bent leads. Theinventive plastic lead frames can also be made transparent to enableultraviolet radiation cure (or other light source cure) of die attachmaterials. This is a distinct advantage over metal alloys which are nottransparent.

[0037] In a third embodiment, the plastic lead frame is formed from aninjection molded or stamped plastic or polymer based sheet of materialto form a lead frame structure. The plastic lead frame structure wouldthen be coated with copper, using electroless copper plating techniqueswell known in the industry. Alternatively, the plastic lead framestructure could be coated with copper using chemical vapor deposition orother plating techniques known in the art. After the copper coating hasbeen applied to the plastic lead frame, the copper may have one or morecoatings or layers of coatings of other conductive metal thereon, suchas a layer of nickel, palladium, silver, gold, other precious metals,etc. In this third embodiment, the resulting low cost plastic lead framehas a conductive layer of copper with suitable electrical and thermalcharacteristics for IC packaging. However, the CTE mismatch between thecopper plated plastic lead frame and the silicon IC die should becomparable to that associated with traditional copper-clad lead frames.

[0038] Plastic lead frames can be manufactured by injection molding,compression molding or by stamping to form complex and intricate shapes.There is no particular limitation on the lead frame shape complexityother than the tools used to perform the injection molding or stamping.Furthermore, by reducing the number of steps necessary to produce theplastic lead frame, relative to a metal lead frame, a lower cost can beachieved.

[0039] Although the present invention has been described with referenceto particular embodiments, the invention is not limited to thesedescribed embodiments. Rather, the invention is limited only by theappended claims, which include within their scope all equivalent devicesor methods which operate according to the principles of the invention asdescribed.

What is claimed is:
 1. A manufacturing method for forming a plastic leadframe structure for use with a at least one semiconductor devicecomprising: forming a plastic lead frame structure from polymericmaterial; and coating said plastic lead frame structure with aconductive material.
 2. The method of manufacturing in claim 1 , furthercomprising: providing a sheet of polymeric material; and forming saidlead frame structure by one of stamping said lead frame structure andetching said lead frame structure.
 3. The method of manufacturing inclaim 1 , wherein said forming said plastic lead frame structurecomprises forming said lead frame by injection molding one of plasticmaterial and polymer material.
 4. The method of manufacturing in claim 1, wherein said forming said plastic lead frame structure comprisedcompression molding one of plastic material and polymer material.
 5. Themethod of manufacturing in claim 1 , wherein said coating comprisesdipping said plastic lead frame structure into a solvent dispersion. 6.The method of manufacturing in claim 5 , wherein said solvent dispersionfurther comprises an intrinsic conductive polymer material.
 7. Themethod of manufacturing in claim 6 , wherein said intrinsic conductivepolymer comprises a polyaniline material.
 8. A manufacturing method fora plastic lead frame structure for use with at least one semiconductordevice comprising one of injection molding a composite plastic materialand an intermixed plastic and a conductive material, said conductivematerial including one of an intrinsic conductive polymer material and apolyaniline material.
 9. A manufacturing method for at least a portionof a semiconductor device package comprising: forming at least oneconductive plastic lead frame having a plurality of lead fingers by onemethod of a method of stamping the conductive lead frame and a method ofetching the conductive lead frame; attaching a semiconductor devicehaving a plurality of bond pads to a portion of said at least oneconductive plastic lead frame; connecting at least one bond pad of saidplurality of bond pads of said semiconductor device to at least one leadfinger of a plurality of lead fingers of said at least one conductiveplastic lead frame; and encapsulating at least a portion of saidsemiconductor device and at least a portion of said at least oneconductive plastic lead frame.
 10. The method of claim 9 , furthercomprising: finishing said semiconductor device package.
 11. The methodof claim 9 , wherein said at least one conductive plastic lead framefurther comprises one of a plastic lead frame structure having a coatingof intrinsic conductive polymer material on at least a portion thereofand a polymer lead frame structure having a coating of intrinsicconductive polymer on at least a portion thereof.
 12. The method ofclaim 9 , wherein said at least one conductive plastic lead framefurther comprises a polymeric lead frame structure having a coating ofcopper on at least a portion thereof.
 13. The method of claim 9 ,wherein said at least one conductive plastic lead frame comprises one ofan injection molded plastic lead frame and a compression molded plasticlead frame.
 14. The method of claim 9 , wherein said at least oneconductive plastic lead frame comprises a stamped lead frame from one ofa sheet of plastic material and a polymer material.
 15. The method ofclaim 9 , wherein said conductive plastic lead frame comprises aconductive plastic lead frame etched from a one of a sheet of plasticmaterial or polymer material.
 16. The method of claim 9 , wherein saidconductive plastic lead frame includes: an intrinsic conductivepolymeric lead frame having a coating of at least one conductivepolymeric material one at least a portion thereof for formation of anintermetallic connection.
 17. The method of claim 16 , wherein saidconductive plastic lead frame includes: a polymeric lead frame having aplurality of coatings of conductive metal layers thereon for formationof an intermetallic connection.
 18. The method of claim 16 , whereinsaid conductive plastic lead frame includes: a polymeric lead framehaving composite metal layers thereon for intermetallic connections. 19.The method of claim 16 , wherein said conductive plastic lead frameincludes: an intrinsic conductive polymeric lead frame having a coatingof at east one conductive metal thereon for formation of a metallicconnection.
 20. The method of claim 16 , wherein the polymericconductive lead frame having coatings of conductive metal layers thereonincludes a coating of copper and a coating of nickel.
 21. The method ofclaim 16 , wherein the polymeric conductive lead frame having coatingsof conductive metal thereon includes a coating of copper, a coating ofnickel, and a coating of palladium.
 22. The method of claim 16 , whereinthe polymeric conductive lead frame having coatings of conductive metallayers thereon includes a coating of copper and a coating of silver. 23.The method of claim 16 , wherein the polymeric conductive lead framehaving coatings of conductive metal layers thereon includes a coating ofcopper and a coating of gold.
 24. A method of manufacturing a circuitcard comprising attaching one or more IC packages to a circuit card, atleast one of said one or more IC packages containing at least oneconductive plastic lead frame formed by one method of a method ofstamping and a method of etching.
 25. The method of claim 24 , whereinsaid at least one of said one or more IC packages each contains aconductive plastic lead frame.